In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by an access node e.g. a radio network node such as a radio access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. The area or cell area is a geographical area where radio coverage is provided by the access node. The access node communicates over an air interface operating on radio frequencies with the wireless device within range of the access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the access nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the access nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising access nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the access nodes, this interface being denoted the X2 interface.
Of the upcoming fifth generation of wireless communication networks 5G, one key design principle currently under consideration is to base the wireless communication network on an ultra-lean design. This implies that “always on signals” from the network should be avoided as much as possible. The expected benefits from this design principle are that the wireless communication network should have a significantly lower network energy consumption, a better scalability, a higher degree of forward compatibility during a Radio Access Technology (RAT) evolution phase, a lower interference from system overhead signals and consequently a higher throughput in low load scenario, and an improved support for user centric beam-forming.
There are principally two sets of mobility procedures considered in both the current LTE standard as well as in the ongoing 5G discussions.
The first set of mobility procedures is denoted ‘Idle Mode Mobility’ and defines how a wireless device which is deemed ‘Idle’, i.e. the wireless device has no ongoing nor a recent data transfer, shall be able to reach the wireless communication network using random access procedures and how to be reachable from the wireless communication network by means of paging procedures etc. In idle mode, the mobility procedures, such as a handover, cell selections or cell reselections, are typically controlled by the wireless device based on a set of rules, e.g. signal level thresholds and carrier frequency priorities, decided by the wireless communication network.
The other set of mobility procedures is ‘Active Mode Mobility’, which has a main task of maintaining the connectivity for an ‘Active’ or ‘Connected’ wireless device, i.e. the wireless device actually has an ongoing or a recent data transfer, as the wireless device moves around in the wireless communication network, and also to handle abnormal cases such as failed handovers, radio link failures etc. In ‘Active Mode Mobility’ the mobility procedures are typically controlled by the wireless communication network, potentially based on measurements from the wireless device.
A complete X2-based intra-Mobility Management Entity (MME)/intra Serving Gateway (S-GW) Handover (HO) procedure for an LTE system is given in 3GPP TS 36.300 “E-UTRA(N) Overall Description; Stage 2” version: V12.4.0 (2014 December).
A key difference between the current LTE mobility procedures as per above, and the upcoming 5G mobility procedures, is that in an ultra-lean system as 5G, as described above, the radio network nodes will prevent themselves from keeping some of the ‘always-on’ signal unlike their counter parts in the LTE system. Instead, the wireless communication network needs to activate the necessary reference signals or beams to measure on only when needed.
The term ‘beam’ used herein is defined in relation with a reference signal (RS). That is, from the wireless device's standpoint a beam is considered as an entity that the wireless device may associate with and is recognized via some reference signals specific to that beam which, in the case of a legacy LTE network, may be the Cell-specific Reference Signals (CRS) of the cell or UE specific reference signals for a specific wireless device. In a wireless communication network with more than one antenna, it is possible for the wireless communication network to form directive antenna radiation patterns, a process which is most often related to as beam-forming. In future wireless communication systems with a large number of antennas, this beam-forming may be very directive and hence provide a very high antenna beamforming gain. In such beam-forming cases, there may be other types of reference signals present, here called simply Beam Reference Signals (BRS) or Mobility Reference Signals (MRS). In all essence however, regardless of the level of directivity of the formed antenna pattern, it is still considered a ‘beam’. Hence, for the simplicity of the exposition, the term ‘beam’ will be used herein.
A service area of a radio network node is a region surrounding the radio network node in which the radio network node is responsible for the active mode mobility related measurements from the wireless device. A wireless device outside such a service area could still be served by the beams from the radio network node but a neighbor radio network node providing radio coverage will be ideally suited for mobility related aspects for the wireless device. Also, such a service area could be a virtual region or could be defined by certain reference signals' coverage. Hence, this 5G concept of service area may be resembled to the coverage area/cell concept of a current LTE system, which has no counterpart in a massively beam-formed system without cell-specific reference symbols being always on.
As stated earlier above, the radio network nodes will prevent themselves from keeping some of the ‘always-on’ signals in order to facilitate mobility for a wireless device being in active mode. Instead, a serving radio network node needs to activate these reference signals at a target radio network node to measure on at the time of handovers. One way of providing such a mechanism is via a position-to-MRS table mapping, wherein an MRS is the reference signal that the wireless device measures on related to a given beam. The position could refer to a geographical position or a so-called ‘radio position’ of the wireless device. The term ‘radio position’ used here could for example be a set of measured quantities such as angle of arrival or experienced path loss of transmissions from the wireless device or signal strength measurements of transmission to the wireless device, in other words a sort of fingerprinting of the radio environment. It could also be based on actively transmitted signals/beacons from some radio network nodes in, or outside, the wireless communication network.
As proposed already, the source radio network node may ask its neighbor radio network nodes to transmit MRS for beams when the source radio network node deems it relevant to do a handover for one or more of its served wireless devices. One such scenario is shown in FIG. 1, where a left radio network node, eNB1, is serving one wireless device, W1, and a right radio network node, eNB2, is serving two other wireless devices, W2, W3,—each in a separate beam. The right radio network node eNB2 is also transmitting one additional MRS/beam in order to facilitate HO for one of the wireless devices it is serving itself—this is however something that is unknown to the left radio network node eNB1. Now, se FIG. 2, the eNB1 may initiate a HO procedure towards the eNB2 for the wireless device W1 served at the eNB1, whereby it will request, with a reference signal request, the eNB2 to start transmitting one or more RSs or beams that can be used for HO related measurements by the wireless device W1 to support the HO procedure, see action 1a. The eNB2 starts the RSs or beams, action 1b in the FIG. 2. Action 1c. The eNB1 sends the wireless device, W1, some measurement control information for enabling measurements. Action 2. The wireless device W1 reports back to the eNB1 with measurement reports. Action 3. The eNB1 makes a HO decision based on the received measurement reports. In case a HO is decided, the eNB1 transmits a handover request to the eNB2, see Action 4. Action 5. The eNB2 performs an admission control and in case the admission control is successful, the eNB2 sends a handover request acknowledgement (Ack) to the eNB1, see Action 6.
This will require resources for signaling and handling the request from the eNB1, leading to an inefficient mobility procedure.